This invention relates to a method and system for improving the fluid blending performance of a pumping system.
There are a number of applications in which fluids must be blended in a highly precise manner. This is particularly true in the case of liquid chromatography in which high pressure liquid metering pumps typically are used. Several liquids, each from separate reservoirs, are blended together in a mixer. The blend then flows to a pump for passage to a chromatograph separating column. Each liquid before entering the mixer first passes through a filter to remove solid particulates.
Fluid blending or proportioning is accomplished by means of plural valves (usually solenoid operated) positioned between each reservoir and the mixer. The valves operate during a valve cycle, typically between 5 and 7 seconds. Each valve is open a certain fraction of the valve cycle time T.sub.v. In the case of four valves A, B, C, D, while the valve for liquid A is open, the valves for the liquids B, C, and D are closed and so on. The fraction of time each valve is open each valve cycle determines the average concentration of that liquid exiting the mixer.
The pump typically consists of two or more positive displacement pump heads, each operating out of phase to maintain a smooth flow profile. If three pump heads are used, each head's piston is 120.degree. out of phase with the two other pistons. For each pump head, the piston displacement is a sinusoidal function of time. The pump period is determined by the pump delivery volume per cycle divided by the volumetric throughput rate. The inlet and outlet of the pump heads are joined at manifolds, which divide the flow at the pump inlet and combine the flow at the pump outlet. The flow into each pump head is pulsatile, i.e., when liquid is drawn into a pump head cavity by retraction of the piston, there is no flow out of the given cavity, and when liquid is forced out of the cavity by the advancing piston, there is no flow into it. This type of operation is obtained by inlet and exit check valves on each pump head to assure that the heads are drawing and delivering fluids at the proper time in the cycle. Under these conditions the three pump heads, for example, produce six flow pulsations into and out of the pump for each pump cycle.
While these positive displacement pumps are quite capable of providing a highly accurate and reproducible flow rate of the pumped fluids, a problem has arisen in that an interaction can occur between the pump cycle and the valve cycle. This interaction results in a very slow, long-term variation in the concentration of the mixed liquids which is too long to be attenuated by the mixer. In the case of liquid chromatography, the blending error of the different liquids should be less than 0.15% and in all cases must be less than 0.5%. Unfortunately, in typical cases, these blending errors far exceed these limits and hence are an undesirable feature of liquid blending systems. This is particularly true when used in those areas such as liquid chromatography in which a high degree of accuracy is required.